The present invention relates to fiber optic systems, and more particularly, is directed to a polarization mode dispersion compensator for long fiber cables.
Polarization mode dispersion (PMD) generally refers to variations in the time delay of a polarized optical signal traveling through an optical transmission system, such as a single-mode optical fiber. PMD arises in an optical fiber because of asymmetries in the optical fiber core, such as core ellipticity created during optical manufacturing and bending stresses resulting from handling the fiber or installing the fiber. Asymmetries in the fiber core cause random changes in the state of polarization (SOP) of optical signals propagating through the fiber. Different SOPs propagate through the optical fiber core at different speeds, resulting in pulse distortion in a transmitted optical signal. Additionally, asymmetries in the core are susceptible to environmental changes, such as temperature or fiber movement, which occur rapidly and further distort the transmitted optical signal.
First order PMD refers to a time delay between two orthogonally polarized principal states of polarization (PSPs). The PSPs are a convenient basis set to describe and characterize each SOP and to evaluate the effects of PMD in the fiber. Using the PSPs as a basis set, each SOP propagating through the fiber is represented as a linear combination of the two orthogonally polarized PSPs. The varying pulse distortion of the SOP is a function of the varying delay between the PSPs.
Second order PMD refers to the frequency dependence of the first order PSP. Second order PMD further distorts the optical signal propagating in the fiber.
Sunnerud et al., “A Comparison Between Different PMD Compensation Techniques”, IEEE J. Lightwave Tech., vol. 20, no. 3, March 2002, pp 368–378, presents several designs for PMD compensators, shown in FIGS. 1A–1F. Sunnerud teaches that feedback is required for effective PMD compensation, but this makes compensation slow. Sunnerud also explains that a large number of control parameters has the advantage of providing multiple degrees of freedom but the disadvantage of complexity. The more degrees of freedom, the longer it takes to arrive at an optimal solution. FIG. 1A shows a polarization controller (PC) used to align the SOP with one of the input PSPs. FIG. 1B shows a PC and fixed time delay, providing two degrees of freedom (DOFs). FIG. 1C shows a PC and variable time delay, providing three DOFs. FIG. 6 shows a variable delay line in free space. A variable time delay enables adjustment of the differential group delay (DGD) (the difference in flight time between the PSPs through the optical fiber core). FIG. 1D shows a double stage compensator with fixed delays, providing four DOFs. FIG. 1E shows a double stage compensator with one fixed delay and one variable delay, providing five DOFs.
PMD can be represented by vectors in three-dimensional Stokes space. Each vector has three coordinates, corresponding to three DOFs. To fully compensate first and second order PMD, at least six DOFs are required. None of Sunnerud's devices can fully compensate first and second order PMD.
Noe et al, “Integrated optical LiNbO3 distributed polarization mode dispersion compensation in 20 Gbit/s transmission system”, Electronics Letters, 15 Apr. 1999, vol. 35, no. 8, pp 652–654, teaches representing PMD as a sequence of three dimensional vectors, each having a length proportional to the DGD of a particular fiber section and differing in direction from its predecessor according to the polarization transformations therebetween. Noe teaches that a PMD compensator mirrors the DGD profile of the fiber, following its vector sequence in reverse direction back to the origin. Noe proposes cascading 73 mode converters to form a PMD compensator, cascading this device with a similar shorter device, and using 246 voltages to control this PMD compensator. However, dynamically adapting the 246 control voltages to the real-time PMD variations in the fiber is extraordinarily complicated and time consuming.
U.S. patent application Ser. No. 09/785,039, filed Feb. 15, 2001, having a common inventor and assignee herewith, presents a feedforward technique of PMD compensation, and is hereby incorporated by reference in its entirety. As shown in FIGS. 2A–2C, a PMD detector, comprising a polarimeter and processor, diagnoses the fibers' PMD parameters, and feeds appropriate control signals forward to a PC and an adjustable delay. However, the proposed devices have only three DOFs and so these cannot fully compensate first and second order PMD.
To provide faster data transmission, it is necessary to further cure the distortion caused by PMD.